Lead has been known to be a toxic element for more than a century. It is especially harmful to children, producing severe adverse effects to their mental and behavioral abilities in direct proportion to the concentration level of lead in their bodies. Governments have limited the concentration of lead in paint and in bulk materials in order to protect the safety of the public, prevent the build up of toxic elements in waste disposals and prevent its reuse in recycling.
Prior teachings of techniques for measuring concentrations of lead in surface-covering layers, such as paint, even when subsequently covered by layers of non-lead paint of unknown thickness and composition, may be found with reference to the following patents: U.S. Pat. Nos. 5,274,688, 5,390,229, and 5,396,529 (all, to Grodzins, and collectively, “Grodzins”), all entitled “Lead Paint Detector,” teach measurement of the concentration of lead in paint on the basis of inducing and detecting fluorescence of the L x-rays of lead. U.S. Pat. No. 5,461,654 (to Grodzins and Parsons, and referred to, hereinafter, as “Grodzins/Parsons”), entitled “X-ray Fluorescence Detector,” teaches a method that provides a measure of the depth of a layer of lead paint beneath one or more layers of paint from which lead is absent. The disclosures of all of the foregoing Grodzins and Grodzins/Parsons patents are incorporated herein by reference. Layer depth, in the prior art, is measured in attenuation units of gm/cm2 and gives no measure of either the density or the elemental composition of the overlayers of paint.
There is an urgent need, unmet by prior techniques, to distinguish, automatically and non-destructively, between toxic elements in a surface layer on an object, possibly buried beneath other material, and similar elements that form one or more constituents of the object's bulk. The distinction must be made if quantitative results are to be obtained.